/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
 var addTwoNumbers = function (l1, l2) {
    const stack1 = []
    const stack2 = []
    while (l1 || l2) { //两链表入栈
      if (l1) {
        stack1.push(l1.val)
        l1 = l1.next
      }
      if (l2) {
        stack2.push(l2.val)
        l2 = l2.next
      }
    }
  
      let carry = 0;
      let ansList = null;
      while (stack1.length || stack2.length || carry !== 0) {//不断出栈
          const s1 = stack1.length ? stack1.pop() : 0;
          const s2 = stack2.length ? stack2.pop() : 0;
          let val = s1 + s2 + carry;
          carry = parseInt(val / 10);//计算进位
          val = val % 10;//计算当前节点的值
          const curNode = new ListNode(val);
          curNode.next = ansList;//向链表前插入新节点
          ansList = curNode;//重新赋值ansList
      }
      return ansList;
  };